How Does Human Reaction Time Affect Car Stopping Distances?

[JDR 007]

Homework Statement

Determine the stopping distances for a car with an initial speed of 85 km/h and human reaction time of 1.0 s for the following accelerations.
(a) a = -4.0 m/s2
m
(b) a = -8.0 m/s2
m

Homework Equations

d=v1+v2/2 *t
a=change in velocity/time

The Attempt at a Solution

I converted 85kh/h into 23.61m/s
I found how many seconds in each instance
(a) 5.9s...I did -4=(0-23.61)/t solved for t
(b)2.95s...same as a
using these times I used (23.61+0)/2 *Time then added 1 for the reaction delay

I got (a)=82 and (b)=47

What am I doing wrong?

 

[atavistic]

Subtract time.